1. Field of the Invention
This invention relates to a mobile communication system, such as an automobile telephone (or carphone), in which a novel method for inferring a cause of a fault upon an occurrence of a radio disturbance (namely, a radio interference on reception) is realized, for use in a communication system which uses electromagnetic waves and is mounted on a mobile unit such as an automobile or a ship.
2. Description of the Related Art
In a communication system mounted on a mobile unit such as an automobile, radio disturbances (namely, radio interferences on receptions) frequently occur owing to the mobility of the unit, which is shielded by a building as shown in FIG. 13. Main causes of radio disturbances are as follows (incidentally, in the following description, names between quotation marks will be simply referred to).
(1) "Blocking", by which the communication system can hardly receive electromagnetic waves, in the case that the mobile unit is completely shielded by a building or the like.
(2) "Shadowing", by which the intensity of an electromagnetic wave is reduced, in the case that the mobile unit is partially shielded by a building or the like.
(3) Interferential "fading", by which the intensity and phase of a transmitted electromagnetic wave become unstable owing to electromagnetic waves reflected from a building or the like.
(4) "Tracking failure", by which a gain corresponding to a desired electromagnetic wave is reduced as a result of a change in the direction of a directional antenna.
(5) "Beam switching", by which the intensity of a transmitted electromagnetic wave is decreased as a result of the relative movement of the mobile unit, on which the communication system is mounted, from the electromagnetic-wave radiation region (namely, the beam coverage) of a base station, from which the communication system receives electromagnetic waves.
In the case when such a radio disturbance occurs, known mobile communication systems take fixed countermeasures against the failure without determining the cause of the failure. Thus, in the case that the actual cause of the radio disturbance is different from expected or inferred causes thereof, when eliminating the failure, sometimes, it all the more takes time to recover communications.
Hereinafter, the configuration of the known mobile communication system will be described by referring to FIG. 14 which is a diagram illustrating the configuration thereof.
As shown in this figure, the known mobile communication system has: an antenna 1 for transmitting and receiving electromagnetic waves; a tracking unit 2 for adjusting the pointing direction (or orientation) of this directional antenna 1 to a direction in which electromagnetic waves arrive; a receiver 3 for demodulating received electromagnetic waves; a transmitter 4 for modulating a signal, which represents data or information to be transmitted, into electromagnetic waves; a handset 5 for receiving a command or instruction from a user; and a control unit 6 for controlling the tracking unit 2, the receiver 3, the transmitter 4 and the handset 5.
Further, as shown in this figure, the controller 6 has a disturbance detection means 61, a countermeasure means 62 and a display means 63.
Next, an outline of an operation of the known mobile communication system will be described hereinbelow by referring to this figure.
When powering the mobile communication system, the control unit 6 indicates a frequency stored therein to the receiver 3 and further issues a signal representing an "instruction to perform a seizing operation" of seizing electromagnetic waves. In FIG. 14, the signal representing this instruction is described as a "receiver control command". Moreover, the control unit 6 issues another signal representing an instruction for the tracking unit 2 to search for the direction in which electromagnetic waves arrive. In FIG. 14, this signal is described as a "search command".
When receiving this instruction, the tracking unit 2 controls the orientation of the antenna 1 in such a manner as to point to a direction in which electromagnetic waves are in good conditions, by monitoring the received signal level of a signal obtained from the receiver 3 during changing the pointing direction of the antenna 1.
Further, when finishing pointing the antenna 1 to the direction in which the receiving conditions are good, the tracking unit 2 notifies the control unit 6 of the completion of the search. This notification is described as "search completion state" in FIG. 14. Thence, the tracking unit 2 controls the antenna 1 so that the antenna 1 keeps pointing to the direction in which the electromagnetic waves are in good condition. This control operation is called "step tracking".
When notified of the completion of the search, the control unit 6 confirms from a "reception (or receive) state signal" that the reception of a signal can be sufficiently achieved. Then, by transmitting a "receiver control command", which represents the "seizing operation", to the receiver 3, the control unit 6 instructs the receiver 3 to perform an ordinary reception operation.
The display means 63 of the control unit 6 is operative to output a display signal to the handset 5 and receive an operation signal from the handset 5. Thus, the control unit 6 controls the handset 5 in such a way that operations can be performed and displays can be produced in the handset 5.
Further, the disturbance detection means 61 judges from the reception state signal whether or not a radio disturbance occurs. If it is consequently judged that a radio disturbance occurs, countermeasures are performed by the countermeasure means 62. Namely, the system is restored from a fault (or failure) state to a normal state thereof by performing predetermined countermeasures.
For example, in the case that the countermeasure means 62 of the control unit 6 of such a system assumes only the "blocking" as the radio disturbance, when the reception state signal monitored by the disturbance detection means 61 of the control unit 6 indicates an occurrence of a radio disturbance, the countermeasure means 62 instructs the receiver 3 to "stop the update of parameters" concerning AGC (Automatic Gain Controller), AFC (Automatic Frequency Controller) and BTR (Bit Timing Recovery) circuit. Further, the countermeasure means 62 instructs the tracking unit 2 to "stop the step tracking." Thus, when the blocking is stopped, the reception is immediately resumed. Although the system is designed in such a manner, the system cannot deal with failures other than the assumed failure.
Hereinafter, an operation, on which attention is focused for accomplishing the present invention, of the known mobile communication system during unreceivable conditions will be described by referring to FIG. 15. Generally, among various disturbances or failures as above described as examples, the control unit 6 is designed in such a manner as to be able to deal with a plurality of causes of disturbances or failures, as illustrated in FIG. 15, by assuming (or selecting) relatively highly probable (causes of) failures among various disturbances described above as examples. Further, the control unit 6 takes up one of the assumed failures arranged in the descending order of the probability thereof and performs countermeasures against the taken-up failure (or disturbance). Then, if the reception conditions are not improved, the control unit 6 takes up the next one of the assumed failures and performs the countermeasures corresponding thereto. Thus, the known mobile communication system performs fixed countermeasures without positively investigating the cause of the radio disturbance.
Namely, at the time of the unreceivable conditions, namely, at the time of an occurrence of a radio disturbance, the control unit 6 stops the updating of the parameter corresponding to the receiver 3 and also stops the step tracking, which is performed by the tracking unit 2, in step 11. This is a countermeasure against the "fading", the "blocking" and the "shadowing".
Subsequently, in step 12, the control unit 6 judges whether or not the radio disturbance is removed within "t" seconds. This time period of "t" seconds is a time period in which the phenomenon such as the assumed "blocking" can last, for instance, 10 seconds. In the case that the system is restored to a normal state, a program of performing this operation advances to step 17. Conversely, if not, the program advances to the next step 13.
Then, in step 13, the control unit 6 instructs the tracking unit 2 to perform the search, and also instructs the receiver 3 to perform the "seizing" operation. This is a countermeasure against the faulty tracking.
Next, in step 14, the control unit 6 judges whether or not the radio disturbance is removed. If not, the program advances to the next step 15. Conversely, if removed, the program goes to step 17.
Subsequently, in step 15, the control unit 6 whether or not adjacent beam are received in good conditions. If the reception conditions are good, the program advances to step 16. In contrast, if poor, the program goes to step 18.
Next, in step 16, the control unit 6 switches a current beam to an adjacent beam (or cell). This is a countermeasure against the "beam switching". Then, the state of the system is changed to a normal receiving state.
Further, in step 17, the control unit 6 instructs the tracking unit 2 to start the "step tracking", and also instructs the receiver 3 to perform a normal receiving operation. Then, the system is changed to a normal receiving state.
Moreover, in step 18, an object of the processing to be performed by the control unit 6 is returned to an initial circuit (or line).
Incidentally, in the case of a known apparatus, which is disclosed in the Japanese Patent Laid-Open No. 3-198438 and is designed in such a manner that a communication system is installed or placed at a fixed position, the investigation of the cause is performed by monitoring variation in the (signal) levels of received electromagnetic waves and by confirming whether or not a reduction in the received signal level is uniformly achieved. Further, this known apparatus performs only countermeasures against the "absorption fading". However, in the case that this known apparatus is mounted on a mobile unit, various radio disturbances occur with rather a high frequency. Thus, this known apparatus cannot be suitably applied to a mobile communication system. Moreover, this known apparatus requires measuring a plurality of electromagnetic waves, so that a plurality of receivers are necessary. Therefore, from the economical viewpoint, this known apparatus is not suited to a compact communication system to be mounted on a mobile unit.
When a radio disturbance occurs, the known mobile communication system does not effectively determine the cause of the radio disturbance but performs fixed countermeasures against the assumed causes by taking up the assumed causes in the predetermined order. Thus, the known system has problems in that the time required to perform countermeasures, which are not suitable for an actual radio disturbance, is wasted and that if unsuitable countermeasures are performed, it all the more takes time to restore the system to a normal state thereof by removing (the cause of) the radio disturbance.
In the case where the cause of the radio disturbance in the known mobile communication system provided with the recovering method illustrated in FIG. 15 is the fact that the mobile unit, on which the communication system is mounted, moves to and comes to belong to an adjoining beam (or cell), the frequency used in the communication system should be changed to a frequency used in a new beam (or cell). In the case of the aforementioned fault recovery processing, no communication can be established for a time period of (t (seconds)+"the time required to the search by the tracking unit 2"). Thus, a mobile communication system of the present invention is provided with mans for inferring the cause of a radio disturbance and means for selecting a suitable countermeasure, thereby achieving the immediate recovery of the system from radio disturbances.
Differently from communication equipment placed at a fixed position, the communication system mounted on the mobile unit is operative to move and thus cannot be installed at a preselected place where obstacles such as buildings are not present. It is, therefore, inevitable that the mobile communication system frequently becomes unable to receive electromagnetic waves owing to obstacles such as buildings and that various radio disturbances occur in the mobile communication system. Such radio disturbances result in increase in transmission error caused on a communication line and in deterioration of communication quality and, in an extreme case, results in a breakdown of communication.
Further, the countermeasures differ according to the kind of the radio disturbance occurring in the system. Moreover, in the case that unsuitable countermeasures are performed, the system cannot recover from the radio disturbance. Furthermore, even when the cause of the radio disturbance is removed, sometimes, it all the more takes time to restore the system to a normal state. Further, in extreme cases, the recovery of the system cannot be achieved.
This invention is accomplished to solve the aforementioned problems of the known communication systems.